Video clipping circuit



I. A. KRAUsE VIDEO CLIPPING CIRCUIT Filed Aug. 30, 1951 074 .1 R/0R A/FT) July 28, 1953 REFERENCE karts/Wm.

D. C. RE6TORER 0R CLAMP REFERENCE POTENTIAL INVENTOR IRVING A. HEAL/3E ATTORNEY Patented July 28, 1953 VIDEO CLIPPING CIRCUIT Irving A. Krause, Jersey City, N. J., assignor to Federal Telecommunication Laboratories, Inc., Nutley, N. J., a corporation of Delaware Application August 30, 1951, Serial N 0. 244,397

1 Claim. (01. 2502'7) This invention relates to video clipping circuits and more particularly to a, synchronizing pulse separator for television circuits.

One of the objects of this invention is to provide a stable clipping circuit for use in video circuitry.

Another object of the invention is to provide a clipping circuit which Will maintain substantially a given clipping level regardless of variations in screen grid voltage, power supply voltage and/or plate current.

An important feature of the invention is the provision, in two opposing legs of a diode bridge clipping circuit, of two electron discharge devices or tubes having substantially equal operating characteristics. By balancing the bridge circuit so that the grid voltage of the input tube is balanced with the clipping level voltage bias of the second tube, the clipping level of the circuit is rendered independent of plate supply voltage changes. With matched tubes the circuit is also independent of the usual Warm-up drift characteristics of the tubes. Once the tubes are adjusted for balance with a given grid voltage, clipping level of the circuit will remain constant or substantially so, even though the tubes warm up or age or the supply voltages vary.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a schematic circuit diagram of a prior art diode clipping circuit; and

Fig. 2 is a schematic circuit diagram of a video clipping circuit according to the present invention.

Before describing the clipping circuit of the present invention, the prior art circuit of Fig. 1 will be described. The circuit of Fig. 1 comprises essentially a bridge network consistin of two like resistors I and 2 and an electron discharge tube 3, the resistance of which is equal to the resistance of a third resistor 4. The resistor l is connected to the anode circuit of the device 3, While resistors 2 and 4 are connected together at point 5 to which an output terminal 6 is coupled through a blockin condenser I. The anode circuit is connected to point 5 through a diode rectifier or crystal 8. A voltage supply is connected as indicated at 9 to the legs of the bridge formed by resistors l and 2. The signal input terminal I 0 is connected to the control grid I I of the tube 3. As shown, the device includes a screen grid I2 supplied with a bias voltage as indicated at l3. The grid II is provided with a D. C. restorer or clamp i l adapted for connection to a given reference potential at I5. The resistor 4 is adjustable so that when the resistance thereof is made equal to the resistance of the vacuum tube 3, the bridge is balanced and there is no output at 6. When the resistance of the vacuum tube 3 is made less than resistor 4, there is no current flow through the diode 8 with its polarity a shown. When the resistance of the vacuum tube is greater than resistor 4, current flows through the diode and a voltage drop is obtained at the output 6.

The instantaneous resistance of the vacuum tube can be controlled by the bias on its grid, that is, the greater the negative bias, the higher the resistance. Depending upon the polarity of the diode 8 and on the value of the resistor 4, the circuit can be used to give an output whenever the grid voltage is greater than or less than a certain definite value. The D. C. restorer M in the grid circuit is used to insure that this value corresponds to a given porch voltage. With a combination video and synchronizing pulse signal, as indicated at l6, applied to the input ID,

a selected bias will provide synchronizing pulses I! only at the output 6.

In actual operation of the prior art circuit of Fig. 1, variations in the filament voltage, screen grid voltage, or plate voltage or the aging of the a tube 3 will each have an eifect on the resistance of the tube thus varying the clipping level of the circuit. While the tube is warming up, for example, which may take several minutes or even hours, a given voltage on the grid corresponding to a given porch voltage may leave the bridge unbalanced. After the tube has wormed up sufficiently, the resistance of the tube may change the bridge circuit toward or further away from balance. The net effect is that the clipping level of the input wave varies objectionably with the changes in the tube parameters.

In accordance with the present invention these variations which affect the operation of the prior art circuit of Fig. 1 do not alter the clipping level of the circuit of Fig. 2. The circuit of Fig. 2 is essentially a bridge circuit comprising two vacuum tubes l8 and I9 as two legs, the tubes selected having substantially identical operating characteristics, and while shown to be pentodes, need not be. The other two legs of the bridge comprise two resistors 20 and 2| of substantially equal value. The resistor 20 is connected to the anode 22 of the tube I8 and the resistor 2| is connected to the anode 23 of tube IS. A diode 24 which may comprise a crystal or other rectifier is connected, on one side, to the inter-connection between the resistor 20 and anode 22 and, on the other side, to the inter-connection between the resistor 2| and anode 23. The screen grids 25 and 26 of the tubes is and I9 are supplied with the same bias voltage as indicated by terminal 21., The other electrodes are connected to ground except for the control grids. The control grid 28 of tube It is connected to the signal input terminal 29 and to a direct current re-- storer or clamp circuit 30 which may be connected to a reference potential at terminal 3i. The control grid 32 of tube leis connected, to a potentiometer 33, one terminal of which is connected to a source of negative voltage as indicated at 34. The output terminal 35-isconnected through a blocking. condenser 38 to the interconnection between resistor 21 and anode 23.

In the operation of the circuit shown in Fig. 2, assuming that resistors 20..- and 2| are equal and the tubes i8 and; 19 are of the same types having substantially identical operating characteristics, the bridge circuit may be balanced at a given clipping level by adjusting the potentiometer 33. When. a signal it is received, the portion of the signal exceeding the clipping level will. be obtained at. the; output terminal 35 as indicated by thesynchronizing pulse l'l. By havingthe two tubes l8 and [9 of substantially identical operating characteristics, any changes in one will be ofiset by like changes in the other. Thus, the two legs oi the bridge circuit will vary identically. thereby maintainin its stabilization.

Once the circuit, is-ad-justed for balance with a givenlgridi voltage, the clipping level of the circuit will remain substantially constant, even though the tubeswarm up er age or. the supply voltagQSe-Vafili;

The circuit as shown in Fig. 2 will function as a, television synchronizin separator. By: reuersing the. polarity of the diode 24 or by inverting the input signal, the circuit will operate as a television synchronizing top clipper.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made by way of example only and not as a limitation to the scope of my invention, as set forth in the objects thereof and in the accompanying claim.

I claim:

A clipping circuit comprising a bridge network having two multi-electrode vacuum tubes of substantially identical operating characteristics connected in circuit as the first and second legs of the network, each tube having anode, cathode, control grid and screen grid electrodes, two resistors of substantially equal value connected to the anodes of said tubes to form the third and fourth legs of the network, a diode connected across the bridge circuit at said anode connections, at source-of anode voltage connected tothe legs containing said resistors, a first source of biasing voltage connected to the screen grids, of bothv said'tubes, a second source of biasing voltage, means connecting the control grid of the. tube of said second leg to said second source of biasing voltage, a signal input and D. C. restorer circuit and means connecting the: control grid of the tube of said first leg to said input circuit.

IRVING A. KRAUSE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,171,918 Dillenburger Sept. 5, 1939 2,463,652 Storm Mar. 8, 1949 OTHER REFERENCES Vacuum Tube Voltmeters by Rider, pp. 57 to- 66, John F. Rider, Publisher, 1116., 1945. 

